On Kundt's tube apparatus

I don't quite understand that apparatus very much. A book said that the disk is at an antinode of motion for the vibrating rod but near a node for the vibrating air column. What does it mean? What can I form if I clamped the rod at the middle?

When clamped in the middle the rod is 1/2 of a wavelength of the sound in the rod. The center (at the clamp) is held stationary, the ends undergo maximum motion, thus are motion anti nodes. This motion of the rod is transfered as variation of pressure to the air in the tube. Thus the end of the rod is a pressure anti node which is the same as a motion node.

You directly measure the wavelength of the sound in the rod and the tube. With knowledge of the speed of sound in the air you can compute the speed of sound in the rod.

Sir "Integral", or to whoever it may concern, in that Kundt's apparatus, what are the two media where the sound travels? When the sound waves travel from a medium to another medium, which will not change, frequency, wavelength or the velocity?